An ultrasonic pressure sensor measures ultrasonic pressure and converts it into an electric voltage signal. These sensors are used widely in a variety of applications ranging from non-destructive testing of materials, marine biology, to medical imaging. In medical imaging, an ultrasonic transducer emits a high frequency pulse into a tissue, and acoustic echoes from the tissue are received by an ultrasonic pressure sensor which is typically implemented on the same transducer. Such a pulse-echo technique can help synthesize a gray-scale tomographic image of tissue's mechanical (or acoustic) properties. One of the most successful ultrasonic imaging devices today is the intravascular ultrasound, or IVUS.
Piezoelectric materials, such as lead zirconate titanate (PZT) or polyvinylidene difluoride (PVDF), have been used to make ultrasonic pressure sensors for many applications. PZT is a ceramic polycrystal with an inherent grain size that makes it difficult to machine and package to small dimensions unless it is attached to a larger substrate. PVDF, which is a polymer, must be electrically poled before it exhibits piezoelectricity, and its sensitivity is lower than that of PZT. Furthermore, the piezoelectric signal is a very small electric voltage that is subject to transmission line loss and electromagnetic interference. For transmission over a long distance such as from a catheter's distal end to its proximal end, the piezoelectric signals typically must be shielded by small coaxial cables and pre-amplified by a chip incorporated near the tip of the catheter. The need to shield and pre-amplify signals makes it hard to construct piezoelectric sensors with a relatively small profile.
Capacitive ultrasonic pressure sensors are being developed by several companies, and they have some interesting properties. They are a type of microelectromechanical systems (MEMS) device fabricated using silicon processing technologies developed in the semiconductor industry. However, these devices are costly to make, and reliability issues are associated with the need to forward-bias the sensing capacitors.
Optical ultrasonic pressure sensors have been proposed and studied by several research groups around the world. In a design described in Beard, et al., “Characterization of a Polymer Film Optical Fibre Hydrophone for the Measurement of Ultrasound Fields for Use in the Range 1-30 MHz: a Comparison with PVDF Needle and Membrane Hydrophones,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 47, No. 1, January 2000, a thin Fabry-Perot etalon is formed on the distal tip of an optical fiber. Laser light is launched into the fiber from a proximal end of the optical fiber and is subsequently reflected back by the etalon and received by a photodetector. Ultrasonic waves interacting with the distal end of the fiber modulates the cavity length of the etalon and causes a change in the reflected light intensity. For this sensor to function properly, the thickness of the polymer constituting the etalon must be controlled to a very high precision during fabrication, which is difficult to do. Furthermore, changes in application environment such as temperature and pressure can significantly alter the properties of the etalon and negatively impact sensor performance. In addition, the fiber optic sensor typically exhibits undesirable ringing or reverberations due to the structure of the sensor that distorts the frequency response. Such reverberations are also problematic in IVUS applications, because it degrades longitudinal image resolution, and it causes a large amount of so-called “ringdown” effect that makes it difficult to image objects close to the surface of IVUS catheter.